The present invention relates to a knock suppression apparatus and method for a multi-cylinder internal combustion engine which can detect knocking in each of the engine cylinders and retard the ignition timing of a knocking cylinder(s) so as to suppress the knocking therein. More particularly, it relates to a knock suppression apparatus and method which can prevent improper or incorrect control on the respective cylinders due to the influences of noise.
FIG. 7 shows a typical conventional knock suppression apparatus for an internal combustion engine which comprises a knock sensor 1 in the form of an acceleration sensor and the like installed on an engine for sensing knocking in the engine, e.g., for sensing engine accelerations due to knocking and generating a corresponding electrical output signal, a knock detecting circuit 2 for identifying signals due to the knocking in the output signal of the knock sensor 1 and generating an output signal of an analog form when knocking is detected, an analog to digital (A/D) converter 3 for converting the analog output signal of the knock detecting circuit 2 into a digital signal, and a microcomputer 4 including an ignition timing controller 5 connected to receive the digital signal from the A/D converter 3 for controlling the ignition timing of the engine in an appropriate manner so as to suppress knocking in the engine. The knock detecting circuit 2 includes a band-pass filter 22 for filtering a particular frequency band of the output signal of the knock sensor 1, a gate circuit 23 operable through the action of a mask signal from the microcomputer 4 to pass specific portions of the output signal of the band-pass filter 22, a BGL circuit 24 connected to receive the output signal of the gate circuit 23 for generating an output signal of a background level, a comparator 25 having a first input terminal connected to receive the output signal of the gate circuit 23 and a second input terminal which is supplied with the output signal of the BGL circuit 24 for making a comparison between these input signals, and an integrator 26 having an input terminal coupled to the output terminal of the comparator 25 and an output terminal coupled to the input terminal of the A/D converter 3 for integrating the output signal of the comparator 25 and outputting an analog signal, the integrator 26 further having a reset terminal coupled to the microcomputer 4 such that it is reset by a reset signal from the microcomputer 4.
The conventional knock suppression apparatus as constructed above operates as follows. First, the knock sensor 1 generates an output signal which is input to the band-pass filter 22 where a specific frequency band thereof is filtered and passed to the gate circuit 23. The gate circuit 23, which has a gate control terminal supplied from the microcomputer 4 with a mask signal containing square pulses occurring at predetermined intervals, operates to mask the output signal of the knock sensor 1 as filtered through the band-pass filter 2 in such a manner that those portions of the mask signal are removed to provide an output signal which contains no pulse at locations corresponding to the respective square pulses. The output signal of the gate circuit 23 is input to the first input terminal of the comparator 25, and at the same time it is imposed on the input terminal of the BGL circuit 24 so that the BGL circuit 24 produces an output signal having a predetermined background voltage level. The background level voltage of the BGL circuit 24 is imposed on the second input terminal of the comparator 25 and compared with the output signal of the gate circuit 23 which is fed to the first input terminal of the comparator 25 so that the comparator 25 generates an output signal when the output signal of the gate circuit 23 is higher than the background voltage level. The output signal of the comparator 25 is fed to the integrator 26 which performs integration to generate an output signal. In this regard, it is to be noted that although not illustrated in FIG. 7, in case of a multicylinder internal combustion engine, a plurality of knock sensors 1 are employed one for each engine cylinder, and the output signals of the respective knock sensors 1 are fed to the integrator 26 in a parallel relation with each other through the band-pass filter 22, the gate circuit 23 and the comparator 25 so that they are respectively integrated by the integrator 26 to provide corresponding output signals. To this end, the microcomputer 4 feeds a reset signal for each knock sensor output to the reset terminal of the integrator 26 at an appropriate timing. The integrated output signal of the analog form of the integrator 26 is then fed to the A/D converter 3 and converted there into a digital signal V.sub.R which in turn is read into the microcomputer 4 where, based on the digital signal V.sub.R, the ignition timing controller 5 calculates an appropriate knock-suppression retard angle .theta..sub.R using the following formula: EQU .theta..sub.R =V.sub.R .times.N
where N is a modification coefficient.
With the above mentioned conventional knock suppression apparatus, however, the hardware components such as the knock detecting circuit 2, etc., takes care of the entire signal processing of determining whether or not there is knocking in the engine. Such a processing requires a rather complicated circuit arrangement and a tremendous load on the part of the hardware components, resulting in high manufacturing costs.